Chel Anderson is a botanist and plant ecologist. She lives here in Cook County and joins us periodically to talk about phenology or what’s going on in the woods right now. Welcome back, Chel.

Anderson: Hi, Jay.

Well, now that winter is officially here—that is, with snow on the ground—let’s talk about all things snow.

Anderson: Snow is one of those miracles of water, right? We are the water planet and we have all these miracles related to water, and one of them, in my opinion, anyway, is snow. You can toss it around, you can pack it into things, you can roll around in it, you can cover up with it—I mean, it’s really, when you stop and think about it as you’re grinding your teeth over having to scrape off the snow on your car again, it’s good to reflect on all the amazing things about snow.

You can throw it at your friends.

Anderson: Yeah, right, games, everything. So many things are possible on snow and with snow, so I thought this is definitely a good topic to try to cover. So, first, let’s start with a couple of basic facts, which I’m sure everyone knows, but bear repeating in this context and that is that snow is a form of water, ice (ice forms at 32 degrees, the magical temperature when it comes to water), and that clouds are water vapor. So, when clouds, or water vapor, encounter temperatures in the air that are at that magical number 32 or lower, snowflakes can form. Something has to happen, because water cannot be a liquid 32 degrees or less. So, in the clouds, often times the response of water vapor to temperatures below freezing is to form snowflakes. The formation of snowflakes is an amazingly dynamic process. If you think about clouds, and maybe what you’ve read about clouds in the atmosphere, it’s not generally a very stable place. Clouds have a lot of air currents swirling around in and through and around them. They have updrafts, downdrafts. They have different temperatures; they are not a uniform temperature. So, the formation of snowflakes is dynamic and depends on all of those environmental conditions. So, a snowflake can begin to form somewhere in a cloud, but then it might encounter a wide range of conditions that are different for the next several hours, and those conditions might cause it to grow into yet a bigger crystal, or they might cause it to melt down completely and become a water droplet again, or to melt down partially and then regrow. So, the story of any individual snowflake could be very long and arduous. We don’t really know. But, what we see is what manages to emerge. Depending on what might happen to be mixed up with the water vapor—dust, dirt—those things can be incorporated into snowflakes as well, and they have an influence on the shape, on the durability of a snowflake, on how quickly it melts once it’s on the ground. All of those environmental conditions described, as well as these kind of foreign particles that might be within the water can have an influence on the formation and the overall life of the snowflake. Where you’re formed as a snowflake will have an influence on your shape. Generally speaking, if you’re a snowflake that is formed high up in the clouds, you’re going to have six-sided hexagonal shape. If you form in the middle, you’re more likely to be needle-shaped. If you’re formed low down, you’re more apt to have a, you know, more smooth, less intricate shape than snowflakes formed higher up in the clouds. Temperature will affect your shape, and temperature might well be associated with where you are in the vertical column of the cloud, but not necessarily, because temperatures can vary quite a bit through the clouds. So, between 32 degrees and 35 degrees you’d look like a beautiful, thin, hexagonal plate. If you formed at 21 to 14 degrees, you could be a hollow column. Again, hexagonal in shape because snowflakes are hexagonal, but you could be a hollow column instead of a plate. At another range of temperatures, you’d form a solid needle, but hexagonal in cross-section. If you form at those temperatures that are really cold, down to two degrees or, you know, the coldest temperatures, then that’s where you’re going to have the most intricate shapes. So, those are going to be those snowflakes that we all go, “wow, how could that be with all those arms and all those smaller arms and branches.” Those dendritic crystals form at the coldest temperatures, so that’s how we get this amazing array of different shapes that we can see over the course of the winter in all of our different snowfalls. Snowfalls can consist of many different shapes; it isn’t necessarily going to be uniform. But, then we’ll have other snowfalls where, boy, pretty much every snowflake is of the same general type. They don’t always look exactly alike. In fact, that’s one of those kind of myths about snowflakes is that no snowflake is exactly the same shape and, chances are, you wouldn’t necessarily be able to find two that are exactly alike, because there so many bazillions and trillions of them. But, there probably is a match somewhere out there. But, from our standpoint, they all do have amazing, different shapes.

I often wondered about that. That would be pushing infinity, I think.

Anderson: Yeah, because the conditions, especially at this time of year, are so unstable, you know, temperatures are hovering just above and below freezing. You’re going to have much more of a chance for there to be all this variability in whether it is snow versus rain or mist. It’s just amazing.

Are snowflakes fairly durable, or do they get buffeted and busted up?

Anderson: Yeah, great question. Snowflakes, really, from the point at which they begin to form until they turn back into water, are constantly being changed. And so, you see one of those incredibly intricate and delicate snowflakes and if it’s January and it’s really cold and it comes down and isn’t being blown around, you can watch them on your jacket for awhile and you can watch them on the surface of the snow for awhile staying just perfect. But, before long, they’re being changed. The sun comes out—boom—it’s just, even in the cold January temperatures, the sun will start to have an impact, especially if there’s a little trace of dust or dirt in there that can absorb a little bit more light than the ice reflects, so that’s going to start to create a little more of a heat source there and start to change. And the wind blows and it wears those arms off and all those physical properties that the snowflake arrives with on the ground are almost immediately beginning to be changed by its environment.

Chel Anderson, botanist and plant ecologist. Thanks for helping us understand what’s going on around us with snow.